Technological Field
The present invention relates to an image forming apparatus and a conveyance control method.
Description of Related Art
In general, an image forming apparatus (such as a printer, a copier, and a facsimile machine) using an electrophotographic processing technique applies (exposes) laser light on the basis of image data to a charged photoconductor drum (image bearing member) to form an electrostatic latent image. In the image forming apparatus, a developing unit supplies toner to the photoconductor drum provided with the electrostatic latent image to visualize the electrostatic latent image to form a toner image. The image forming apparatus further primarily or secondarily transfers the toner image to a sheet and heats and pressurizes the sheet by a fixing nip of a fixing unit to fix the toner image on the sheet. The image forming apparatus is also provided with registration rollers that correct positional deviation of the sheet in a width direction, on an upstream of a transfer section that transfers the image to the sheet (for example, see Japanese Patent Application Laid-Open No. 2014-133634 (hereinafter, referred to as PTL 1)).
By the way, the image forming apparatus has a problem of occurrence of a phenomenon (sub scanning obliqueness) in that a conveyance direction of the sheet is passed obliquely in a sub scanning direction due to misalignment from the registration rollers to the fixing nip through the secondary transfer nip. In addition to the case of the misalignment, the sub scanning obliqueness is likely to occur when there is a difference between the diameters at opposite ends of the rollers in the sheet width direction (sub scanning direction) due to a durability issue or the like. A long sheet with a long size in the conveyance direction is easily affected, and the sub scanning obliqueness often occurs. The sub scanning obliqueness leads to a poor image due to deviation, distortion, or the like of the image transferred at the transfer section, and a technique for reducing the sub scanning obliqueness is demanded.
To counter the problem, the registration rollers can be displaced to control skew correction to eliminate the sub scanning obliqueness on some level. On the other hand, although the control can correct a front-end side of the sheet in the conveyance direction, a rear-end side of the sheet is still tilted. The distortion remains between the front-end and the rear-end of the sheet, and it is difficult to eliminate the sub scanning obliqueness.
A technique of registration displacement control is also described in PTL 1, in which a line sensor is disposed between the registration rollers and transfer rollers, the line sensor detects a sheet edge position while the transfer rollers convey the sheet, and the registration rollers are moved based on the detection result.
However, in the registration displacement control described in PTL 1, the longer the length of the sheet in the conveyance direction is, the more the displacement operation of the registration rollers needs to be repeatedly performed. Therefore, there is a problem that the irradiation time of the line sensor increases, and the life of the line sensor is reduced.